Mechanical fittings are often coupled to another component (a "mating component") through the use of threads disposed on the fitting and the mating component. Typically, the threads of the fitting and the mating component are securely joined by applying a torque to the fitting. The inadvertent application of excessive torque can damage the threads of the fitting or the mating component. Such damage can prevent the effective mating of the fitting and the mating component, and may necessitate repair or replacement of the fitting or the mating component. Torque-limiting devices are utilized to prevent this type of damage. Torque-limiting devices facilitate the transfer of a limited amount of torque between a torque-applying device, e.g., a wrench or a socket, and the fitting.
Torque-limiting devices typically limit torque in one of two ways. One type of torque-limiting device relies exclusively on friction. In particular, one or more sliding surfaces within these types of devices generate a frictional force as the surfaces translate in response to the applied torque. The frictional force limits the amount of torque that can be transferred to the fitting.
Another type of torque-limiting device relies on mechanical interference. Specifically, a component within the device is placed in contact against another component in a manner that facilitates the transfer of torque between the components. A spring-loaded mechanism holds the components in contact while the applied torque is below a particular value. The spring-loaded mechanism allows the components to separate when the applied torque exceeds a predetermined value, thereby preventing the transmission of torque levels above that value.
Commonly-used torque-limiting devices have a number of substantial drawbacks. For example, the limiting torque produced by such devices usually changes after repeated use of the device. This characteristic is due to wear of the friction-producing surfaces, or permanent stretching of the springs that facilitate the torque-limiting function. Hence, an optimum limiting torque is difficult to maintain throughout the useful life of such devices.
In addition, the springs utilized in spring-loaded devices are usually large, and cannot be integrated into the device in a manner that minimizes the overall dimensions of the device. Furthermore, spring-loaded devices usually have a high parts count, and rely on relatively complicated mechanical interactions between their individual component parts. These characteristics add size and weight to the device, and tend to decrease reliability. Furthermore, spring-loaded devices usually incorporate a number of relatively fragile components. Hence, spring-loaded devices are not particularly suited for use in high-vibration, high-temperature, or corrosive environments.
Both friction-based and spring-loaded torque-limiting devices are susceptible to contamination by foreign materials. Specifically, friction-based devices are particularly susceptible to contamination by grease, oil, and other commonly-used lubricants, as these types of materials alter the coefficients of friction on the friction-producing surfaces. Spring-loaded devices can be affected by foreign materials that collect on the torque-transferring surfaces within the device. In particular, the presence of such materials can reduce the effective contact area of the surfaces, and thereby alter the limiting torque.
Thus, a need exists for a torque-limiting device of compact size and low complexity. In addition, it is desirable that the device be suitable for use under harsh operating conditions, e.g., in high-vibration or high-temperature environments. Furthermore, the device should be capable of producing a limiting torque that changes minimally over the useful life of the device. Also, the device should be capable of satisfactory operation in the presence of common contaminants such as lubricating materials. The present invention is directed to these and other goals.